vhost/crypto: fix descriptor processing

vhost/crypto: fix descriptor processing

copy_data was returning a pointer to an increased (off by one) descriptor.
Subsequent calls to copy_data in the library were then failing.
Fix this by incrementing the descriptor only if there is some left data
to copy.

Fixes: 4414bb67010d ("vhost/crypto: fix build with GCC 12")
Cc: stable@dpdk.org
Reported-by: Jakub Poczatek <jakub.poczatek@intel.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Jakub Poczatek <jakub.poczatek@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>